1. Field of the Invention
The invention relates to an organic electroluminescent device. In particular, the invention relates to an organic electroluminescent device which is suitable for a so-called top emission type for extracting emitted light from a side opposite to a substrate on which the device is formed.
2. Description of the Related Art
An organic electroluminescent device (so-called organic EL device) is a device of a self-luminous type having a light emitting layer made of an organic compound between an anode and a cathode and is watched as a device capable of realizing a large-area display device of low-voltage driving. In a display device using such an organic electroluminescent device, by fabricating a circuit provided with a thin film transistor (TFT) for driving the organic electroluminescent device into an active matrix type formed on the same substrate as the device, it is possible to devise to achieve high functionalization of the device. In the active matrix type, since a necessary current can be injected and kept for every pixel, there is theoretically no limitation in a screen size. Thus, the active matrix type is expected to be applied to display devices of 5 inches or larger, too.
In manufacturing a display device of an active matrix type, an organic electroluminescent device is formed on a substrate on which a thin film transistor has been previously formed (so-called TFT substrate) in a state that it is connected to the subject thin film transistor. For that reason, what the organic electroluminescent device is made of a so-called top emission type for extracting emitted light from a side of an upper electrode as an opposite side to the substrate is effective in ensuring an aperture ratio of pixel.
In the organic electroluminescent device of a top emission type, by configuring an upper electrode formed so as to have a reverse polarity against a lower electrode which becomes an anode or a cathode, emitted light generated within the device is extracted from a side of the upper electrode.
Here, an indium-tin-oxide (ITO) film which is usually used as a transparent conductive film has a high work function as approximately 5 eV so that while the ITO film is suited for an anode for which a material with high work function is suitable, it is not suitable for a cathode using a material with low work function.
For that reason, with respect to the configuration in which the upper electrode is a cathode (namely, a transparent cathode), there is proposed a configuration in which an electron injection layer made of a mixture of a metal with low work function such as cesium (Cs) and an electron transporting organic material is stacked between an organic light emitting layer and an upper electrode made of a transparent conductive film, thereby enhancing electron injection properties. However, a number of transparent conductive films represented by the foregoing ITO are made of a metal oxide and formed by a sputtering method using, as a process gas, argon (Ar) or oxygen (O2). For that reason, at the time of film formation of a transparent conductive film, there was involved such a problem that the metal-containing electron injection layer as a base is decomposed and oxidized to cause a lowering of electron injection properties, an increase of driving voltage, the generation of current leakage, a lowering of life, and so on.
In order to prevent this problem from occurring, there is proposed a configuration in which a metal layer made of an alkali metal or an alkaline earth metal and a layer made of a mixture of such a metal and an electron transporting organic material are stacked in this order between the organic light emitting layer and the upper electrode made of a transparent conductive film. According to such a configuration, it is described that a phenomenon in which the metal layer is oxidized at the time of film formation by sputtering of a transparent conductive film, whereby the device is deteriorated is prevented from occurring; and that the metal layer keeps high injection efficiency, whereby luminous efficiency is improved (see JP-A-2004-296410 (especially, see FIG. 1 and paragraph [0009]) (Patent Document 1)).
On the other hand, even in the case where the upper electrode is an anode, the base layer (hole injection layer) is damaged at the time of film formation by sputtering of a transparent conductive film, whereby hole injection properties are lowered. Then, there is proposed a configuration using, as a hole injection layer, a material containing an inorganic material and having electron accepting properties, for example, V2O5. According to such a configuration, it is described that the damage of the hole injection layer at the time of film formation by sputtering of a transparent conductive film which becomes the upper electrode is suppressed, whereby high hole injection efficiency can be realized (see JP-A-2005-32618 (especially, see FIG. 1 and paragraphs [0044] to [0045]) (Patent Document 2)).